Ecotoxicology and risk assessment

 

Characterization and management of the solid residue from biogas production in the context of the polycyclic aromatic hydrocarbons content and toxicity

Biochars from sewage sludge – physico-chemical and ecotoxicological properties and application

in environmental protection



Sewage sludge contains many valuable substances that positively affect soil properties. Its high content of phosphorus and nitrogen as well as of micro- and macronutrients is the main reason why it is used in plant cultivation. Nevertheless, due to the presence of various contaminants (trace metals, persistent organic pollutants and emerging contaminants) and pathogens, the possibility of using sewage sludge as fertilizer is questioned more and more often. As a result of this, efforts are undertaken more and more frequently to find safer solutions for the disposal of sewage sludge than its direct incorporation into soils. Biochar, which is defined as fine grained carbonizate obtained during pyrolysis of biomass or wastes, can be an answer to the current problems related to sewage sludge reuse.

The aim of the work was to evaluate the effect of pyrolysis of sewage sludge on the physico-chemical and ecotoxicological properties of obtained biochar. Also the possibility of its application was evaluated. Sewage sludges of varying organic matter content were pyrolyzed at temperatures of 500, 600 and 700 oC. The obtained materials were characterized in terms of their composition and physico-chemical as well as their surface and thermal properties. In the obtained materials the total concentration of polycyclic aromatic hydrocarbons (PAHs), which are a group of contaminants that are ever present in sewage sludges and which should be subject to special treatment due to their mutagenic and carcinogenic properties, was determined. Moreover, the content of bioavailable fraction of contaminants, that is, the one responsible for the toxicity and mobility of contaminants, in biochars was evaluated. 

Additionally, the trace metal content was determined before and after pyrolysis as well as the ecotoxicological parameters test towards plant (Lepidium sativum), bacteria (Vibrio fischeri) and crustacean (Daphnia magna). Also the sorption capacity of biochars (alone and with the presence of soil) towards PAHs was assessed in the context of the possibility of using them for water purification and contaminant immobilization in soils contaminated by such compounds. With increasing treatment temperature, the pH, ash content and macro- and micronutrient content increased. The biochar aromaticity and hydrophobicity also increased. On the other hand, the pyrolysis yield, percentages of H, N and O, molar ratios, polarity of biochars, and crystallite size decreased. The direction of the changes in the content of elemental carbon (C) and in surface area was dependent on the type of sewage sludge (Zielińska et al., 2015). It was found that for some properties of biochars produced could be determined on the basis of an analysis of the properties of the initial sewage sludge. Sewage sludges conversion to biochar significantly reduced the content of PAHs (from 8- to 25-fold depending on pyrolysis temperature and kind of sludge) (Fig. 2; Zielińska and Oleszczuk, 2015a). The pyrolysis of sewage sludges caused also a significant reduction of their toxicity towards the test organisms. In turn, after pyrolysis an increase was noted for trace metals content (Pb, Cd, Zn, Cu, Ni and Cr). However, the bioavailable fraction of trace metals was at lower level in biochars then in sewage sludges. The biochars produced from the sewage sludges contained several times fewer bioavailable PAHs than the corresponding sewage sludges. The pyrolysis of sewage sludge significantly improved its sorption capacity towards PHE and PYR (Zielińska and Oleszczuk, 2015b). An increase in the sorption potential of the soils as a result of application of the sewage sludge-derived biochars was observed.

The studies demonstrate that sewage sludge-derived biochar can be an interesting and above all safer alternative to sewage sludge that the materials obtained can be used to purify water contaminated with such compounds or to immobilize soil contaminants.

Selected references:

  1. (1)Zielińska A., Oleszczuk P., Charmas B., Skubiszewska-Zięba J., Pasieczna-Patkowska S. Effect of sewage sludges properties on the biochar final characteristic. J. Anal. Appl. Pyrol. 112 (2015) 201-213.

  2. (2)Zielińska A., Oleszczuk P. The conversion of sewage sludge into biochar reduces PAHs content and ecotoxicity but increases heavy metals content. Biomass and Bioenergy 75 (2015a) 235–244. 

  3. (3)Zielińska A., Oleszczuk P. Effect of sewage sludge pyrolysis on freely dissolved polycyclic aromatic hydrocarbon (PAH) concentration in biochars produced in different temperatures. Chemosphere 153 (2016) 68-74.

  4. (4)Zielińska A., Oleszczuk P. Bioavailability and bioaccessibility of polycyclic aromatic hydrocarbon (PAH) in historically contaminated soils after application of sewage sludge-derived biochars. Chemosphere 163 (2016) 480-489.

  5. (5)Zielińska A., Oleszczuk P. Evaluation of sewage sludge and slow pyrolyzed sewage sludge-derived biochars for adsorption of phenanthrene and pyrene. Biores. Technol. 192 (2015b 618–626.

Anaerobic fermentation of organic materials, taking place in biogas plants, is a process used for energy generation. At the same time, it is one of the main strategies permitting reduction of the amount of organic wastes. The primary raw materials used in the production of biogas include manure, plant residues, wastes from the food industry, municipal wastes, food and agricultural wastes and sewage sludge (Figure on the left).

The number of biogas plants, both in Poland and in Europe, is growing. Experts forecast that in Poland by the year 2020 the increase of the number of agricultural biogas installations will con- tinue at the rate of several dozen percent a year. This will be related with a simultaneous increase of the amounts of generated RBP. In near future that will be a major problem associated with utilization of wastes of this type. Therefore, research concerning the properties of the material generated, with particular emphasis on its effect on the environment, is a necessity.

Residues from biogas production (RBP) are relatively new materials, which may be an interesting resource for the improvement of soil fertility. RBP have a high content of organic matter and other nutritional components that are the necessary for the proper development of plants and other soil organisms. However, the application of the RBP may be limited due to  the high salinity or high mobility of nutritional components, as well as the content of heavy metals or polycyclic aromatic hydrocarbons (PAHs). Therefore RBP needs a comprehensive estimation of their toxicity before applying them to the soil on a large scale.

Samples of RBP obtained from six biogas production plants with varied biogas production methods were analysed. The samples with and without separation on solid and liquid phases were investigated. The physicochemical properties of the RBP, heavy metals content (Cr, Cu, Ni, Cd, Pb i Zn), polycyclic aromatic hydrocarbons and toxicity on bacteria (Vibrio fischeri, MARA test – 11 different strains), collembolans (Folsomia candida) and two plant species (Lepidium sativum and Sinapis alba) was investigated.

An especially negative effect on the tested organisms whereas was noted for the liquid phase after separation. In many cases, RBP without separation also showed unfavourable effects on the tested organisms. In most cases, no ecotoxicological effect was observed for solid phase after separation for tested organisms. The solid phase after separation presented the most favorable properties between all investigated RBP. Therefore, it can be a potential material for the improvement of soil properties and for later use in agriculture.


Selected references:

  1. (1)Stefaniuk M., Oleszczuk P. Characterization of biochars produced from residues from biogas production. J. Anal. Appl. Pyrol. 115 (2015) 157–165.

  2. (2)Stefaniuk M., Bartmiński P., Różyło K., Dębicki R., Oleszczuk P. Ecotoxicological assessment of residues from different biogas production plants used as fertilizer for soil. J. Hazard. Mat. 298 (2015) 195-202.

  3. (3)Stefaniuk M., Oleszczuk P. The total and freely dissolved polycyclic aromatic hydrocarbons content in residues from biogas production. Environ. Pollut. 208 (2016) 787–795.

  4. (4)Stefaniuk M., Oleszczuk P., Bartmiński P. Chemical and ecotoxicological evaluation of biochars produced from residues from biogas production. J. Hazard. Mat. 318 (2016) 417–424.

Another way of  RBP development is their conversion to biochar (BC), during the pyrolysis process (Figure on the right). The pyrolysis of RBP to BC may cause carbon immobilization and higher nutrient stability. Therefore, the production of RBP-derived biochar is an interesting solution. Nevertheless, before such a method of RBP usage, the properties of produced biochar should be examine not only in terms of the potential benefits, but also the potential risk to the environment. Therefore, it is particularly important to determine the content of pollutants in biochars and evaluation of their ecotoxicological properties.

In this research the physiochemical and ecotoxicological characteristic of RBP derived from different biogas plant, as well as the self-produced biochar made of chosen RBP it the temperature ranges from  400 to  800°C is provided. Samples of RBP originated from biogas plant with varied biogas production methods  (mesophilic or thermophilic anaerobic fermentation condition) and separated (solid and liquid fraction) or not residues (non-separated fraction).

In terms of  physiochemical and ecotoxicological properties the best material both for direct application to the soil and for biochar production substrate turns out to be the solid RBP after separation. Less favorable properties possess the liquid and non-separated fraction of RBP and BC produced from non-separated materials.

Effect of biochar aging on their properties and interaction with organic and inorganic contaminants in context of their toxicity (NCN Opus, 2017/25/B/NZ8/02191) (PI: Anna Siatecka)

The main aim of research will be to determine the persistence and intensity of biochar-contaminant interactions as affected by simulated biochar aging processes in the context of their impact on the ecotoxicological properties of biochars.

The main research hypothesis is the assumption that aging processes of biochar will be influence changes in the availability of contaminants, which will be determine the toxicity with respect to different groups of organisms. The extent of changes in the availability of contaminants and hence toxicity  will be depend on the aging method.

More information about project will be provided soon.